Z. PINCZÉS
Institute o f Geography, Kossuth Lajos University, P.O. Box 4, H-4010 Debrecen, Hungary
ABSTRACT
Erosion on the agricultural land o f Hungary is associated with showers in summer and it takes place on the thawed surface o f frozen soil in winter. The magnitude o f soil loss is similar in the two seasons. Soil erosion in winter takes diverse forms. In their dynamics these processes on slopes resemble to each other and represent boundary cases with the common characteristic that the thawed top soil is transported by gravitation over the frozen subsoil. The medium o f transport is water, the amount o f which controls the way o f transport and the resulting feature. A classification is made by the amount o f meltwater, and the following types were distinguished: gelifluction, soil flow, sheet wash and rill erosion.
INTRODUCTION
The extent and forms o f soil degradation in Hungary are essentially determined by two main factors: relief conditions and climate. The country lies at the deepest point of the Carpathian Basin, the marginal parts are, however, occupied by mountainous and hill regions, which together constitute a significant proportion of the country’s area (32 %). The hill region consists exclusively of loose marine sandy and clayey sediments, volcanic tuff and are agriculturally utilized on ploughland, in vineyards and orchards. Thus, loess due to erosion is the highest in these areas, as a consequence of tillage. On the other hand, soil degradation also takes place on flatland. Here it is mostly the sand surface which is especially damaged, primarily in spring when the poorly developed vegetation is yet unable to give protection against erosion by wind.
From the agriculturally cultivated area of the country 2,319,840 hectares are eroded, including
weekly eroded (less than 30 % of the profile removed) 37.4 %, moderately eroded (30-70 % removed) 38.5 %,
strongly eroded (more than 70 % removed) 24.1 %.
of about 50,000,000 m of soil is eroded every year from slopes. Out of this amount 40.000. 000 m3 is deposited at the feet of the slopes or the meadows and pastures lying along streams and rivers. A further 1,000,000 rn fills up the beds of the water courses, 8.000. 000-10,000,000 m3 accumulates in sandbars along major rivers or is transported to the sea. As experts estimate it, the lack of proper soil conservation involved an annual erosion loss in Hungarian agriculture amounts to an equivalent of 1,000,000 tons of wheat. It has been proved by calculation that with the application of the appropriate soil-conserving cultivation methods the soil loss could be reduced from 50,000,000 m3 to ca. 12,000,000 m3 (Stefanovits, 1963, 1964).
The other factor influencing soil erosion is climate. Hungary’s climate is of transitional character, as the country lies on the border of the West-European (oceanic) and East-European (continental) provinces. This transitional character also implies that soil erosion hazard is high in both winter and summer. In winter groundfrost and meltwater, in summer heavy showers are responsible for soil erosion. Our measurements show that in the particular years the extent of erosion damage can vary with seasons according to weather conditions, whereas on the long run it is of similar scale in winter and summer.
FORMS OF SOIL EROSION IN WINTER
Erosion in winter and especially at the end of the season can have special and varied forms, which are primarily the consequences of the freezing of soil. The depth of groundfrost is influenced by altitude above sea level, exposure, the physical properties of the soil, vegetation, etc. but on the average the soil surface is frozen to 30 cm depth.
EROSION ON STEEP SLOPES
The steep slopes with no vegetation cover are highly sensitive to winter frosts if they are markedly soaked through in the autumn. Erosion due to frost action can be well observed every winter along roads, ditches, edges of silt and loess embankments.
As a result of autumn and winter precipitation the walls of the above-mentioned formations wet to a few millimetres or centimetres depth. Due to groundfrost ice crystals and needles grow and separate the frozen layer from the unfrozen horizon. In consequence of the sucking effect of frost the ice needles formed continue to grow and the separated rock flake is detached from the wall. On warming up, especially on sunny early spring days the ice needles melt and the separated rock fragments fall down and pile up at the foot. Overgrowth by vegetation hinders the action o f frost.
The parts overgrown with roots erode slowly, whereas adjacent surfaces not protected by vegetation show a faster decline. Consequently, the surface of steep slopes (wall)
t
grows uneven. With the progress of denudation the steepness of the wall is reduced by the gradual accumulation of material at the foot.
M ASS MOVEMENTS ON SLOPE
The common feature of mass movement types is that the thawed soil is moved over the frozen subsoil as a consequence of gravitation. The amount of water controls the way of transportation and the resulting form. On the grounds of the amount of meltwater the following groups of forms can be distinguished:
1. Gelifluction
Gelifluction is the slow slide of saturated loose soil material over frozen subsoil.
The most important criterion of this type of movement is the presence of frozen subsoil which prevents the penetration of meltwater from the surface layer deep into the lower horizons. In consequence, the saturated top layer assumes such a state of viscosity and plasticity that it loses its stability and, as a result of gravitation slides downslope. The distribution of this process is highly limited in Hungary. It is confined, in optimal cases, to a few weeks, days or perhaps only hours at the end of winter, when conditions characteristic of periglacial climate may develop. This process can be detected from several characteristic traces: the undulating slope surface, the tilted tree trunks may be indicators of such movement. Such conditions can primarily evolve on the northerly slopes of the mountains, where there is a possibility of deep, enduring groundfrost. The moisture content of the moving material may be as high as 40 %. The movement extends over the whole slope material covering the bedrock. In this sense gelifluctional movement may also have a serious role in valley evolution.
2. Talus creep
This form also falls within the group of gelifluction. It evolves on fine-grain soil interwoven with plant roots. In this very slow type of movement only a thin layer is affected. For this reason no observation is possible, only tongues of soil overcreeping one another can indicate its occurrences.
3. Mudflow
In cases when the supply of water is so plentiful that it results in the overmoistening of soil, gelifluction gives way to a process where water is preponderant. This process is mudflow and is confined primarily to agriculturally utilized areas. The process starts, as a rule, on inflectional line of the slope or below this line, and it can arise on thick groundfrost and at the time of fast thaw (thawing can also be promoted by rain). These latter conditions can be present only on slopes with northern and north-eastern exposure.
The water content of the thawed soil surface is around 50 % because of deep, impermeable groundfrost. The plastified material with high water content becomes fluent. The material which starts flowing down may assume various forms. Initially it appears as a linear
feature. A particular mudflow may reach a width of a few dozen centimetres and length of several metres or some tens of metre. In the top segment, because of the loss of material due to flow, depressions of 4-10 cm come into being. On the middle and lower segments the mudflow creeps onto the surface and continues to flow elevated by a few centimetres.
Mudflow is a more rapid movement than gelifluction. Part of the runoff water, as it flows faster than dilute mud, gets free from the flowing material, and preceds it in its movement.
Consequently, mud loses its water content, its motion is slowed down, the material is piled up on the slope or moves on at a lower speed as gelifluction. Linear mudflows usually appear on the slope in groups. In the course of their evolution they often adjoin one another and surface lowering becomes areal. This may result in the erosion of several centimetres of soil surface over a large area.
4. Sheetwash
In the evolution of this process it is essential that an excess amount of water be present at the time of thaw. In this case there is water preponderant. Sheetwash erosion plays a significant role at the beginning of the period of thaw. The frozen subsoil has not yet thawed, melting of ice only begun on the surface. Owing to the frozen subsoil the runoff meltwater cannot cut into the surface, therefore, it can run only along the surface and washes off the slope. The grains of soil freed by thaw or loosened by water are drifted away by water and transported to the feet of the slope. The process of sheetwash is limited in time. When thaw is prolonged and a larger amount of material is washed off by runoff water, sheetwash erosion gives way to mudflow (Pinczés, 1971a, 1971b, 1979).
5. Rill erosion over frozen subsoil
This is the form of erosion most frequently emerging at spring thaws. It occurs in areas built up of silty, loessy, clayey sediments and is completely absent in sand regions.
Its evolution is also determined by weather conditions. Favourable factors are: hard winter, deeply frozen subsoil, sudden thaw. As our measurements show, the difference in erosion damage can be two to threefold in different years depending on the variation of weather. The size of the rills evolved is also influenced by the exposure of the slope.
They are rare on southerly exposed slopes, since on these slopes snow melts intermittently during the winter, thus, the thickness of the snow cover has gradually decreased by the time spring comes (the depth of groundfrost is also smaller) and the meltwater of the remaining thin snow blanket does not involve a considerable erosion hazard.
The danger of erosion is highest on slopes with westerly exposure, and it is twice or three times less on easterly and northerly slopes. The thickness of the thawed soil layer determines the shape of rills. The rills deepen downwards only as far as the frozen subsoil.
If the thawed layer is thin, the rills grow laterally and broaden. When a thicker layer of soil is thawed then the rills will be deeper.
Our measurements caried out over a period of several years have proved that erosional damage due to meltwater may reach 100 m loss of soil per hectares. The observations also turn attention to an interesting phenomenon. The data obtained from different places in different years show that the amounts of eroded material as calculated
from the size of the rills and that of the material sedimented at the foot are not identical.
The sedimented material is twice or sometimes nearly five times as much as has been calculated from the dimensions of the rills. This fact suggests that at that time of end-of-winter thaws various soil degradation processes take place simultaneously and exert a joint effect. As to our example, the sheetwash action of meltwater was much more significant than rill erosion or perhaps the erosional damage due to mudflow. The latter two eroded the surface between the rills and acted in a way that can be called „invisible erosion”. In fact, there takes place, besides the well-observable linear erosion, a very strong overland erosion. Our observations testify that winter soil erosion takes place in a highly varied way. These processes frequently occur simultaneously even in a particular area, act together and may result in very marked surface denudation (Pinczés and Boros, 1967).
R E FER E N C E S
Kerényi, A. (1981). A study o f the dynamics o f drop erosion under laboratory conditions. Erosion and Sediment Transport Measurement. Proceedings o f the Florence Symposium, June, 1981. IAHS Publ. 133.
Pinczés, Z. and Boros, I. (1967). Eróziós vizsgálatok a Tokaji-hegy szólóterületein (L’érosion dans les régions viticoles du mont á Tokaj). Acta Geographica Debrecina 12-13,308-325.
Pinczés, Z. (1971a). Erosion forms and erosion control in the vine-lands o f the Tokaj Mountains. International Geographical Union European Regional Conference, Budapest, Hungary, 60.
Pinczés, Z. (1971b). Die Formen der Bodenerosion und der Kampf gegen sie im Weingebiet des Tokajer Berges. Acta Geographica Debrecina 10,63-70.
Pinczés, Z. (1979). The effect o f groundfrost on soil erosion. Seminar on agricultural soil erosion in temperate non-Mediterranean climate, Strasbourg, 107-112.
Stefanovits, P. (1963). A magyar talajeróziós térképezés alapjai (The fundamentals of Hungarian soil erosion mapping).Orsz. Mezőgazd. Könyvtár és Dokumentációs Központ, MÉM Információs Központja (in Hungarian).
Stefanovits, P. (1964). Talajpusztulás Magyarországon. Magyarázatok Magyarország eróziós térképéhez (Wasting of the soil in Hungary). OMMI, Budapest, 7, 58 (in Hungarian).